Fiber optic communication systems have typically been used in broadband configurations to trunk large amounts of information over long distances. The large bandwidth and relatively low loss characteristics of optical fiber have made it useful for the efficient transport of large information bandwidths at relatively low cost. Lately, fiber optic subsystems are also finding more use in systems that were typically designed for electronic radio applications. CATV, Radar and some cellular and Personal Communication Service (PCS) subsystems now employ fiber optic links that allow operators to extend the coverage region or move more TV channels over wider distribution areas. The impending multimedia revolution, will involve the delivery of larger bandwidths to residential sites via radio subcarriers. The use of fiber is likely to increase greatly when these new services place enormous bandwidth demands on existing service delivery systems.
For clarity we define three types of optical modulation that are addressed in this document: 1. Baseband Modulation in which the signal modulated onto an optical carrier via a laser diode or external modulator, is lowpass in nature. 2. Subcarrier Modulation in which the baseband information is first modulated onto an electrical or radio frequency carrier; the modulated electrical or radio frequency carrier is then used to modulate an optical carrier. The information from the perspective of the optical carrier is itself a (bandpass) modulated carrier. 3. Subcarrier Modulation with Harmonic Frequency Translation in which the modulation of a radio frequency carrier is modulated as in subcarrier modulation above but it is then modulated onto an optical carrier in a fashion that produces harmonics of the subcarrier signal about the optical carrier. Upon detection of the optical signal, a desired harmonic of the original radio subcarrier is isolated in a bandpass filter. See Generation and Transmission Of FM and p/4 DQPSK Signals at Microwave Frequencies Using Harmonic Generation and Opto electronic Mixing in Mach-Zehnder Modulators, Tom Young, Jan Conradi, Wayne Tinga and Bob Davies, IOOC 95, June 1995 and Generation and Transmission Of FM and p/4 DQPSK Signals at Microwave Frequencies Using Harmonic Generation and Opto electronic Mixing in Mach-Zehnder Modulators, Tom Young, Jan Conradi and Wayne Tinga, IEEE Transactions on Microwave Theory and Techniques, vol. MTT 44, pp 446-453, March 1996.
The net result of this exercise is optical transport of a radio subcarrier with an additional frequency translation to some higher frequency at the optical detector output. The benefit of this method is that the information signal may be modulated onto the optical carrier at a lower cost by using lower frequency radio components.Baseband modulation is typically applied in broadband data transmission whereas subcarrier modulation is typically used for distribution of CATV, Cellular and other media that employ radio or radio compatible transmission and signal multiplexing. Baseband and subcarrier optical fiber distribution systems are subject to distortion related to loss, noise, and nonlinearities in both the fiber and the modulation and amplification devices. One of the more deleterious forms of nonlinear distortion is that due to chromatic dispersion. Chromatic dispersion in optical fiber is typically characterized to a first order by a linear group delay parameter. The group refractive index of the fiber at optical frequencies near a given optical carrier frequency varies approximately linearly with wavelength or optical frequency about the carrier. This finite linear group delay imposes a quadratic phase rotation across the signal frequency band. While dispersion has an undesired effect on any signal of finite bandwidth it is of particular concern in optical systems where the modulated signal is also contaminated with an unwanted phase or frequency modulated component which is prevalent when optical sources such as semiconductor lasers are directly modulated, and when various external modulators are used. See Fumio Koyama, Kenichi Iga, "Frequency Chirping In External Modulators" Journal of Lightwave Technology, Vol. 6, No 1, pp 87-93, January 1988.
Approaches currently used to reduce the effects of chromatic dispersion include: (1) reversing the effects of chromatic dispersion in the optical domain, (2) reversing the effects in the electrical domain after optical detection and (3) reducing the transmission bandwidth of the optical signal on the fiber. The first is based on purely optical methods where the effects of group velocity dispersion are reversed while the signal is still in the optical domain. Adding dispersion compensating fiber in the transmission path is one common approach. Other optical methods include compensation by differential time delay of the upper and lower sidebands of the modulated signal, see A. Djupsjobacka, O. Sahlen, "Dispersion compensation by differential time delay," IEEE Journal of Lightwave Technology, vol. 12, no. 10, pp. 1849-1853, October 1994; spectrally inverting the signal at the midpoint of the transmission path, see R. M. Jopson, A. H. Gnauck, R. M. Derosier, "10 Gb/s 360-km transmission over normal-dispersion fiber using mid-system spectral inversion," Proceedings OFC'93, paper PD3, 1993; and pre-chirping the transmitted signal in an external modulator, see F. Koyama, K. Iga, "Frequency chirping in external modulators," IEEE Journal of Lightwave Technology, vol. 6, no. 1, pp. 87-03, January 1988 and A. H. Gnauck, S. K. Korotky, J. J. Veselka, J. Nagel, C. T. Kemmerer, W. J. Minford, D. T. Moser, "Dispersion penalty reduction using an optical modulator with adjustable chirp," IEEE Photonics Technology Letters, vol. 3, no. 10, pp. 916-918, October 1991.
The second approach, in which dispersion effects are reversed in the electrical domain, is based on coherent transmission and heterodyne detection followed by equalization in the electrical domain. It is important to note that a double sideband (DSB) signal must be heterodyne detected if the signal is to be compensated electrically. If homodyne detection were attempted with a DSB signal, the upper and lower sidebands would overlap upon detection and the phase information would be lost and the higher modulation frequencies severely attenuated or distorted through cancellation of sideband components. Some techniques used or proposed for post-detection equalization include microstrip lines, see K. Iwashita, N. Takachio, "Chromatic dispersion compensation in coherent optical communications," Journal of Lightwave Technology, vol. 8, no. 3, pp. 367-375, March 1990; microwave waveguides, see J. H. Winters, "Equalization in coherent lightwave systems using microwave waveguides," Journal of Lightwave Technology, vol. 7, no. 5, pp. 813-815, May 1989. and fractionally spaced equalizers, see J. H. Winters, "Equalization in coherent lightwave systems using a fractionally spaced equalizer," Journal of Lightwave Technology, vol. 8, no. 10, pp. 1487-1491, October 1990.
The third approach is to modify the transmission format where the baseband signal spectrum is compressed. These types of approaches, which reduce the transmission bandwidth required on the fiber to transmit a given bit rate, are generally implemented by modifying the line code format in order to reduce the effective bandwidth required to transmit or receive the data, see K. Yonenaga, S. Kuwano, S. Norimatsu, N. Shibata, "Optical duobinary transmission system with no receiver sensitivity degradation," Electronic Letters, vol. 31, no. 4, pp. 302-304, February 1995, and G. May, A. Solheim, J. Conradi, "Extended 10 Gb/s fiber transmission distance at 1538 nm using a duobinary receiver," IEEE Photonics Technology Letters, vol. 6, no. 5, pp. 648-650, May 1994.
The generation, transmission and detection of single sideband (SSB) signals has been used in the RF and microwave regions of the electromagnetic spectrum to reduce the bandwidth of the signal by a factor of two, by sending either the upper or the lower sideband. Generation and transmission of SSB optical signals using a Mach-Zehnder modulator is shown in M. Izutsu, S. Shikama, T. Sueta, "Integrated optical SSB modulator/frequency shifter," IEEE Journal of Quantum Electronics, vol. QE-17, no. 11, pp. 2225-2227, November 1981 and R. Olshansky, "Single sideband optical modulator for lightwave systems," U.S. Pat. No. 5,301,058, 1994. A dispersion benefit accrues from a single sideband signal due to the fact that the transmitted signal spectrum has been reduced by a factor of two.
A more significant advantage of optical SSB transmission is that upon detection, particularly if the signal is coherently detected, a dispersed baseband signal is produced where the information of the relative arrival time of the various signal frequencies remains as part of the electrical output signal and hence the fiber dispersion can be compensated in the electrical domain after detection. This advantage is similar to that for heterodyne detection of DSB signals, but with SSB transmission and detection, the signal can be homodyned directly to baseband using carrier signal added either at the source or at the receiver and thus it can be directly detected with the phase or delay information of the transmitted signal preserved.
An early integrated optical SSB modulator using optical filtering techniques was described in K. Yonenaga, N. Takachio, "A Fiber chromatic dispersion compensation technique with an optical SSB transmission in optical homodyne detection systems," IEEE Photonics Technology Letters, vol. 5, no. 8, pp. 949-951, August 1993, where integrated optical structures were used to generate single sideband tones for narrowband applications. In K. Yonenaga, N. Takachio, "Dispersion compensation for homodyne detention systems using a 10 Gb/s optical PSK-VSB signal," IEEE Photonics Technology Letters, vol. 7, no. 8, pp. 929-931, August 1995, a single sideband optical modulator was described for the purpose of transmitting two or more optical signals with different optical carrier frequencies on a single fiber. The purpose of transmitting the signals in a single sideband format is to permit these optical carrier frequencies to be spaced as closely as the maximum modulation frequency. Further work on generation of SSB optical signals with a Mach-Zehnder modulator is found in Olshansky, U.S. Pat. No. 5,301,058.
In another technique, disclosed in co-pending application Ser. No. 08/738,573, now U.S. Pat. No. 5,880,870 assigned to the same assignee, a single Mach-Zehnder modulator is driven by Hilbert Transform pairs in two different ways to yield a vestigial sideband. In one case the Mach Zehnder is driven in "Hartley Fashion" with Hilbert Transform pairs of a baseband digital signal applied to the individual modulator arms. In the other case the modulator is driven in hybrid fashion with linear combinations of the baseband signal Hilbert Transform pairs to simultaneously achieve an optical signal that is amenable to simple envelope detection and vestigial sideband generation and transmission. The use of Hilbert transform pairs to generate single sidebands is disclosed in Gordon B. Lockhart, "A Spectral Theory for Hybrid Modulation", IEEE Transactions on Communications, Vol. COM 21, No. 7, July 1973.
Baseband and subcarrier optical fiber systems may be improved if either some or all of the upper or lower optical sideband band could be removed from the optical signal before transmission. For Intensity Modulated--Direct Detection (IMDD) systems each sideband carries redundant information in terms of what is required to demodulate the information signal at the destination so there is no requirement for double sideband optical transmission. What remains after sideband removal is a narrow band signal that even with FM contamination is greatly reduced in effective bandwidth and dispersion distortion.
For subcarrier modulation, which typically has a much lower information bandwidth per subcarrier, the separation of upper and lower optical signal sidebands after optical modulation may be quite large. Upon detection the upper and lower optical sidebands are essentially collapsed onto each other and any phase distortion that the signal encountered on the fiber will be imposed on the detected subcarrier signal as cross band phase distortion. See Basil W. Hakki, "Dispersion of Microwave-Modulated Optical Signals", Journal of Lightwave Technology, Vol. 11, No 3, pp 474-480, March 1993.